Introduce StartAfterFunc

pmuxlib/proc.go

@@ -48,6 +48,11 @@ type ProcessConfig struct {
 	// NoRestartOn indicates which exit codes should result in the process not
 	// being restarted any further.
 	NoRestartOn []int `yaml:"noRestartOn"`
+
+	// StartAfterFunc will cause the starting of the process to be blocked until
+	// this function returns. If used with RunProcessOnce and an error is
+	// returned, RunProcessOnce will return that error.
+	StartAfterFunc func(context.Context) error
 }
 
 func (cfg ProcessConfig) withDefaults() ProcessConfig {
@@ -104,6 +109,12 @@ func RunProcessOnce(
 
 	cfg = cfg.withDefaults()
 
+	if cfg.StartAfterFunc != nil {
+		if err := cfg.StartAfterFunc(ctx); err != nil {
+			return -1, err
+		}
+	}
+
 	var wg sync.WaitGroup
 
 	fwdOutPipe := func(logger Logger, r io.Reader) {